In recent years, protection of passengers from automobile collisions has been acknowledged as an aspect of utmost importance for automobiles, and hopes are increasing for suitable materials exhibiting excellent high-speed deformation resistance. For example, by applying such materials to front side members of automobiles, the energy of frontal collisions may be absorbed as the materials are crushed, thus alleviating the impact on passengers.
Since the strain rate for deformation undergone by each section of an automobile upon collision reaches about 10.sup.3 (l/s), consideration of the impact absorption performance of a material requires knowledge of its dynamic deformation properties in a high strain rate range. Because it is also essential to consider at the same time such factors as energy savings and CO.sub.2 exhaust reduction, as well as weight reduction of the automobile, requirements for effective high-strength steel sheets are therefore increasing.
For example, in CAMP-ISIJ Vol. 9 (1996), pp.1112-1115 the present inventors have reported on the high-speed deformation properties and impact energy absorption of high-strength thin steel sheets, and in that article it was reported that the dynamic strength in the high strain rate range of about 10.sup.3 (l/s) is drastically increased in comparison to the static strength in the low strain rate of 10.sup.-3 (l/s), that the strain rate dependence for deformation resistance varies based on the strengthening mechanism for the material, and that TRIP (transformation induced plasticity) steel sheets and DP (ferrite/martensite dual phase) steel sheets possess both excellent formability and impact absorption properties compared to other high strength steel sheets.
Furthermore, Japanese Unexamined Patent Publication No. 7-18372, which provides retained austenite-containing high strength steel sheets with excellent impact resistance and a method for their production, discloses a solution for impact absorption simply by increasing the yield stress brought about by a higher deformation rate; however, it has not been demonstrated what other aspects of the retained austenite should be controlled, apart from the amount of retained austenite, in order to improve impact absorption.
Thus, although understanding continues to improve with regard to the dynamic deformation properties of member constituent materials affecting absorption of impact energy in automobile collisions, it is still not fully understood what properties should be maximized to obtain steel materials for automotive members with more excellent impact energy absorption properties, and on what criteria the selection of materials should be based. Steel materials for automotive members are formed into the required part shapes by press molding and, after usually undergoing painting and baking, are then incorporated into automobiles and subjected to actual instances of impact. However, it is still not clear what steel-strengthening mechanisms are suitable for improving the impact energy absorption of steel materials against collisions subsequent to such pre-deformation and baking treatment.